A conventional motor casing typically comprises an end plate supporting a brush holder and a cup-shaped yoke connected to the end plate. The cup-shaped yoke is typically formed by a deep drawing process. FIG. 14 shows such an example. A radial outer flange 31a is formed at an open end of a cup-shaped yoke 31. The flange 31a is formed at the same time as forming the yoke 31 by a deep drawing process.
The circumferential wall 31b of the yoke 31 that supports permanent magnets 32 is required to have a certain thickness so that a magnetic path having an adequate cross sectional area may be achieved. Therefore, the material for forming the yoke 31 is required to consist of a plate member having a corresponding thickness. When such material is subjected to a deep drawing process, the bottom portion that defines a bearing support portion 31c is stretched, and is given with a reduced wall thickness upon completion of the deep drawing process. However, the radial flange 31a would not be stretched to such an extent during the deep drawing process, and is given with a wall thickness d similar to that of the circumferential wall upon completion of the deep drawing process. Therefore, the thickness of the radial flange 31a is greater than desired, and this adds to the weight of the yoke without providing any benefits.
However, there is an ever growing demand to reduce the weight of a motor, and it is desired to minimize the weight of a yoke without unduly complicating the manufacturing process.
Japanese patent laid-open publication 11-089123 discloses a double-walled yoke that combines a tubular inner yoke for supporting permanent magnets and a cup-shaped deep-drawn outer yoke having a relatively small thickness. Thereby, only the part of the yoke that defines the magnetic path for the permanent magnets is given with an increased thickness while the remaining part of the yoke is given with a reduced thickness. Therefore, the weight of the yoke can be minimized without compromising the magnetic performance of the yoke.
According to this previously proposed yoke, the inner yoke and outer yoke are integrally combined apparently by press fitting the inner yoke into the outer yoke. However, a press fitting process may involve seizure and/or deformation of the inner yoke unless the inner and outer yokes are prepared at a very high precision, and a considerable amount of effort and cost are required to carry out the press fitting process for a yoke of this type in a successful manner.